I spent two batches at Recurse Center in New York during 2012. At the time I was self-taught and fairly new to programming. There were three main themes that emerged during my time at RC. After my first batch, I wrote a reflection on things I wanted to learn.
Clojure and Clojurescript, to get better at Lisp. Pseudo live-image generator (with Alexander Clare), Clojurescript library for live data-binding (with Conner Petzold), Clojurescript reflection capabilities (with Dustin Getz, Zach Allaun, Mary Rose Cook, Julien Fantin and James Keene), and a Domain Specific Language for logic gates.
Go, to learn about concurrency. Chat server (with Senthil Arivudainambi), Monte Carlo simulation (with David Peter), Websockets abstraction for handling events (with David Peter), Fractal terrain generator (with David Peter).
Static site generators, to scratch my own itch. One in Clojure (with Zach Allaun), and one in Bash.
After attending RC, I also helped them interview over 500 prospective applicants (both conversational and technical code pairing).
If you want to become a better programmer, I highly recommend applying.